Optical elements composed of various kinds of optical glass are widely used for optical pickup lenses for discs such as CD, DVD, BD and HD-DVD, and used for image pickup lenses installed in portable telephones. Recently, demand to the optical element composed of such the optical glass is rapidly grown accompanied with the fast spreading of optical-disc recorder/player and portable telephone with camera. Consequently, rising in the production efficiency and lowering in the cost of the optical element is required.
In the optical disc recorder/player, increasing in the recording density and the volume of recorded information is rapidly progressed accompanied with the recent progress in information technology. It is necessary to raise the driving speed of the optical pickup lens by reducing the weight of the optical pickup lens for rapidly reading the information in the large capacity optical disc. Therefore, optical glass having low specific gravity is required.
Such the optical element can be produced by a press molding method. A method is known as a type of the press molding method in which a preformed glass having designated weight and shape is previously prepared and the preformed glass is reheated together with a mold and pressed on the mold to form the lens; hereinafter such the method is referred to as a reheat-press method.
As another type of the production method, the following method is known; hereinafter referred to as a drop-press molding method. In the method, a drop of molten glass is formed through a dropping nozzle, and the drop is dropped onto a mold and then is pressed for forming the lens during the solidification by cooling the molten glass drop. By this method, repetition of heating and cooling of the mold is not necessary and the formed glass can be directly produced from the molten glass drop. Therefore, the time for once forming the glass can be very shortened and higher production efficiency can be expected compared with the reheat-press method. Therefore, the method is remarkable.
As the optical glass to be used in the production of the optical element by the press molding method, having a refractive index nd (the refractive index for helium d line having a wavelength of 587.56 nm) from 1.56 to 1.63, and Abbe number vd from 56 to 63, documents such as U.S. Pat. No. 5,744,409 and JP-A Nos. 2003-89543 and 2004-137145 propose SiO2—B2O3—R2O—R′O—La2O3 type optical glass, where R is an alkali metal and R′ is an alkali-earth metal.
However, the optical glasses described in U.S. Pat. No. 5,744,409 and JP-A Nos. 2003-89543 and 2004-137145 have very high thermal expansion coefficient such that the average thermal expansion coefficient α within the temperature range from 100 to 300° C. is not less than 90×10−7. Therefore, such the glasses tend to cause a problem of occurrence of defect such as breaks and cracks because of heat shrinkage caused in a press-molding process. Particularly, in the case of the drop-press molding method, such the defects are considerably caused because the molten glass is rapidly cooled during pressing the drop dropped onto the mold.
Moreover, each of the optical glasses described in U.S. Pat. No. 5,744,409 and JP-A Nos. 2004-137145 has a viscosity suitable to drop a molten glass drop such as a viscosity of from 5 to 30 poises at a very high temperature. For instance, a temperature of 1,300° C. or more is required for making the viscosity of the molten glass drop to 5 poises. Therefore, the molten glass drop at such the high temperature causes considerable degradation of the mold when the drop contacts with the mold and the lifetime of the mold is much shorted, which is a problem.
Moreover, the optical glass described in JP-A No. 2003-89543 has high specific gravity such as 2.98. Consequently, such the glass hardly corresponds to the requirement of weight reduction of the optical element. Besides, the drop of the molten glass is released and dropped from the dropping nozzle when the amount of the molten glass accumulated at the end point of the dropping nozzle is arrived at the designated weight. Accordingly, a problem is also caused that the glass drop having large volume can be difficultly dropped when the specific gravity of the optical glass is high.